They are obviously very closely related because they have all the same atoms, but there's no way to lay one exactly on topic of the other - just like your hands! Chemists call pairs of molecules like these isomers (pronounced ICE-oh-merz), and they are compared to hands in their names! One is called D (from the Latin word dextro, for right) and the other is L (from the Latin levo, for left).
Many molecules that are important to life come in two (or more) isomer forms, but very often, only one of them occurs naturally. Sugars, for example, are almost all in the D-form, but amino acids, which make up proteins, are almost all L. Spearmint leaves contain a molecule called L-carvone; caraway seeds (the seeds in rye bread) contain D-carvone - and you can even taste the difference!
The difference between these isomers can be very serious, though. In the late 1950's and early 1960's, a drug called thalidomide was often given to pregnant women because it helped relieve nausea. However, many babies born to women taking this drug had serious birth defects or died. In all, over 10,000 children in almost 50 countries were affected. Interestingly, thalidomide was never approved for use in the United States. A doctor named Frances Oldham Kelsey, who worked for the Food and Drug Administration, was suspicious of thalidomide, and she refused to allow it to be prescribed. Her decision saved many babies. So what does this have to do with mirror molecules? Thalidomide has two isomers, and it turns out that only one of them causes the birth defects. If the drug had not contained both of them, many lives could have been saved.
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